1. Field of the Invention
This invention relates to a method of and device for improving the quality of fresh concrete in a rotary mixer drum of a concrete mixer truck or of a concrete mixing plant. Cold or warm air is selectively and continuously delivered into a rotary mixer drum of a concrete mixer truck or into a stationary mixer drum of a concrete mixing plant during, just before, or just after unloading the fresh concrete and also just before washing the inside of the rotary mixer drum. This is done to maintain the temperature of the fresh concrete at a required temperature, and to delay a hardening time of the fresh concrete. Subsequently, water is sprayed into the rotary or stationary mixer drum, after having finished discharging the remaining fresh concrete, to wash it away before it has hardened.
2. Description of the Prior Art
Concrete is an artificial stony material which is used for foundations, etc. made by mixing cement, sand and broken stones, etc. with water, and allowing the mixture to harden. Quick hardening and solid concrete is indispensable as a building material. These solid and quick hardening properties of concrete may be disadvantageous sometimes.
As shown in FIG. 1, a conventional rotary mixer drum 20 is rotatably mounted on a chassis 10a of a mixer truck 10 with a hydraulic motor 12 which is linked to a gear 16 by a roller chain 14, the gear 16 being provided at a central portion of a bottom portion of the rotary mixer drum 20.
There has been an annoying problem in the conventional concrete mixer truck 10 and in the concrete mixing plant when the chemical reaction of the mixed fresh concrete advances to adhere on the plurality of rotary blades and around the inner periphery of the rotary concrete drum, etc. and hardens thereon. The concrete, once adhered and hardened on the rotary blades and the inner periphery of the rotary mixer drum 20 of the concrete mixer truck 10 and in the concrete mixing plant, is hard to scrape off.
It has been customary in the art that the remaining fresh concrete in a bottom portion of the rotary mixer drum 20 of a concrete mixer truck 10 in a concrete mixing plant is washed away by water after the fresh concrete has been discharged or unloaded out of the rotary mixer drum 20.
The temperature in the rotary mixer drum 20 rises to a range between 40-50 degrees Celsius under the blazing sun in summer, resulting in a shortened hardening time and delivery range of the fresh concrete and also decreasing the strength of the hardened concrete. In addition, when the mixed fresh concrete is unloaded out of the conventional rotary mixer drum 20 of the concrete mixer truck 10, the temperature in the rotary mixer drum 20 rises suddenly so that the remaining fresh concrete hardens in shorter time.
Moreover, inasmuch as the temperature in the rotary mixer drum 20 itself is rather high, the fact that the fresh concrete is not continuously agitated and gathers at the bottom portion of the rotary mixer drum 20 before it is unloaded, further accelerates the adhesion and hardening of the fresh concrete. To this end, even when water is sprayed into the rotary mixer drum 20 immediately after the fresh concrete has been unloaded, the remaining fresh concrete, once hardened, cannot be washed away. Accordingly, it is required that an operator must enter the rotary mixer drum 20 to scrape or tear away the hardened concrete with a hammer.
Washing by water is usually carried out immediately after the fresh concrete has been unloaded from the batcher concrete mixer, but even if washing by water is carried out, a certain amount of the adhered concrete builds up. In practice, it is necessary for the operator to scrape the adhered concrete off of the mixer drum of both the concrete mixer truck and of the concrete mixing plant once a week.
It is very dangerous for the operator to enter the mixer drum of the concrete mixer truck 10 and of the concrete mixing plant in order to scrape the adhered and hardened concrete off the mixer drum. Regrettably, a number of fatal accidents are reported every year when the rotary mixer drum 20 is carelessly rotated without knowing that the operator is within the rotary mixer drum 20 mounted on the concrete mixer truck. In order to avoid such an accident, a safety device with a sensor for detecting when the operator is within the rotary mixer drum 20 has been proposed. However, washing by water and the scraping operation by the operator within the rotary mixer drum 20 would still be required, leaving open the possibility for accidents.
It is stipulated under the JIS (the Japanese Industrial Standards) that the time between mixing fresh concrete to beginning to unload the fresh concrete must be within 90 minutes. When the time has passed 90 minutes, the temperature of the mixed fresh concrete has risen, causing the fresh concrete to deteriorate due to the high temperature and shortening the delivery range. Accordingly, the fresh concrete, thus mixed, is likely to be refused by a user as the product is below the standards. It is said that concrete mixing and transporting are a battle against time.
In another embodiment, cooling systems or refrigerators using liquified gas have been proposed to cool the fresh concrete temporarily so that the strength of the placed concrete is increased, cracking of the deposited concrete is prevented and the quality of the concrete is maintained. Among them are many cooling systems in which the fresh concrete is directly cooled prior to transportation. One of the conventional cooling systems is characterized in that the fresh concrete is cooled at a building site or a construction field.
Mounted on the concrete mixer truck 10 is another conventional cooling system in which liquified gas is directly injected into the fresh concrete (see Japanese Patent Publication No. 2295-07/1986, not-examined). It should be appreciated, however, that liquified gas is very expensive, and although it may be suitable for a temporary cooling of fresh concrete, a large amount of liquified gas is required for continuous cooling during transportation.
Another embodiment has been proposed, wherein flakes of crushed ice are mixed into fresh concrete in a rotary mixer drum, but there is a disadvantage. When the flakes of crushed ice remain in the fresh concrete at the time of placing the fresh concrete, openings or gaps occur in the fresh concrete as a result of the crushed ice, thus deteriorating the strength of the hardened concrete.
On the other hand, when the rotary mixer drum 20 is exposed to the open air in a cold season, dropping the temperature in the rotary mixer drum, the strength of the mixed fresh concrete decreases. In order to eliminate this disadvantage, a countermeasure has been worked out in which the mixture ratio of the cement is changed in accordance with the temperature of the open air. However, the mixing adjustment is very troublesome, and the material costs increase as well. Adhesion and hardening of the remaining cement in the rotary mixer drum also occurs in the cold season even though the temperature is low.